The reaction of cytochrome oxidase with oxygen in the fission yeast Schizosaccharomyces pombe 972h-. Studies at subzero temperatures and measurement of apparent oxygen affinity

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Abstract
Cytochrome a3 in whole-cell suspensions of the fission yeast S. pombe reacted in the reduced form with CO to give a photodissociable CO complex with absorption maxima at 429, 543 and 591 nm in CO-liganded reduced-minus-reduced difference spectra. Other CO-bound hemoproteins, cytochromes P-420 and P-450, were not photodissociated under the conditions employed. Measurements of the rates of reassociation of CO with cytochrome a3 after flash photolysis over the temperature range from -101 to -109.degree. C gave a value for Eact. [energy of activation] of 28.6 kJ/mol. Between -94 and -106.degree. C, O2 reacted with cytochrome oxidase in intact cells to give an oxygenated intermediate (compound A). At -70.degree. C compound A was converted into a second spectrally distinct intermediate (compound B). Electron transport, indicated by the oxidation of cytochromes a + a3 and cytochrome c, did not occur until the temperature was raised to -50.degree. C. At room temperature cytochrome oxidase was oxidized to 50% of its steady-state concentration by 0.35 .mu.M-O2.